1. The Field of the Invention
The present disclosure relates in general to 3D subsurface imaging of different; physical properties of geological formations and/or man-made objects using geophysical data acquisition systems with multiple sources and/or sensors.
2. The Related Technology
Geophysical surveys are widely used in mineral, hydrocarbon, geothermal and groundwater exploration, in-situ mining, hydrocarbon, geothermal and groundwater resource monitoring, unexploded ordinance (UXO), improvised explosive device (IED), tunnel, and underground facility (UGF) detection, geosteering, bathymetry mapping, ice thickness mapping, and environmental monitoring.
To provide economical reconnaissance of subsurface geological formations and/or man-made objects, geophysical data acquisition systems with multiple sources and/or sensors are often deployed from fixed arrays and/or moving platforms such as vessels, wireline devices, bottom hole assemblies (BHA), vehicles, airplanes, helicopters, airships, and unattended aerial vehicles (UAV).
Land geophysical surveys from at least one data acquisition system include various configurations of EM systems such as natural source EM systems (e.g., raagnetotellurics, audiomagnetotellurics) and/or controlled source electromagnetic (CSEM) systems (e.g., induced polarization, spectral induced polarization, controlled source magnetotellurics, controlled source audiomagnetotellurics, time-domain sounding), and/or various configurations of gravity and/or gravity gradiometry systems, and/or various configurations of magnetic and/or magnetic gradiometry systems.
Marine geophysical surveys from at least one vessel include various configurations of EM systems such as towed streamer EM systems, controlled source electromagnetic (CSEM) systems, and marine magnetotelluric (MMT) systems, and/or various configurations of towed streamer reflection seismic systems such as narrow azimuth (NAZ) systems, wide azimuth (WAZ) systems, and extra wide azimuth (XWATS) systems, and/or various configurations of gravity and/or gravity gradiometry systems, and/or various configurations of magnetic and/or magnetic gradiometry systems.
Borehole geophysical surveys from at least one borehole include various configurations of EM systems such as cross-borehole EM systems, and/or various wireline deployed induction logging and/or tensor induction logging devices for formation evaluation, and/or various bottom hole assembly (BHA) deployed induction logging and/or tensor induction logging devices for logging-while-drilling (LWD), measurement-while-drilling (MWD), and/or imaging-while-drilling (IWD), and/or various wireline gravimeter and/or gravity gradiometer systems, and/or various wireline magnetometer and/or magnetic gradiometer systems.
Airborne geophysical surveys from at least one aircraft include various configurations of EM systems such as natural source EM systems (e.g., AFMAG, ZTEM, AirMt, airborne magnetotellurics) and/or controlled source electromagnetic (CSEM) systems (e.g., DIGHEM, RESOLVE, GEOTEM, MEGATEM, SPECTREM, TEMPEST, VTEM, AEROTEM), and/or various configurations of gravity and/or gravity gradiometry systems (e.g., FALCON, Air-FTG), and/or various configurations of magnetic and/or magnetic gradiometry systems.
Geophysical surveys may acquire large volumes of data covering very large areas. For example, airborne geophysical surveys from fixed wing aircraft typically acquire 500 line km of data each day, and airborne geophysical surveys from helicopters typically acquire 200 line km of data each day. Airborne geophysical surveys typically contain multiple survey lines that aggregate as hundreds to thousands of line kilometers of multiple channels of geophysical (e.g., gravity, magnetic, EM) data measured every few meters and cover an area hundreds to thousands of square kilometers in size.
Subsurface imaging is a discipline inclusive of geophysical imaging, migration and/or inversion that reconstructs a physical property volume image of subsurface geological formations and/or man-made objects. The state of the art in methods of subsurface imaging has been discussed by Zhdanov, 2002, and Zhdanov, 2009.
As an example, the state-of-the-art in airborne EM interpretation is based on various 1D methods such as conductivity depth images (CDIs), conductivity depth transforms (CDTs) layered earth inversions, laterally constrained layered earth inversions, and spatially constrained layered earth inversions. These methods cannot reliably or accurately capture the geological complexity of the 3D subsurface conductivity.
As another example, the state-of-the-art in gravity and magnetic interpretation is based on various 3D inversion methods. The survey area is discretized as a 3D earth model where each cell of the model is characterized by a uniform physical property such as density and/or magnetic susceptibility and/or magnetization. The gravity and/or magnetic fields and/or their sensitivities are predicted from the entire 3D earth model. This requires considerable computer memory and processing resources. Large gravity and/or magnetic surveys are reduced to subsets (also called “tiles”) which are independently inverted using software such as GRAV3D and/or MAG3D from the University of British Columbia Geophysical Inversion Facility. After each subset (or tile) has been inverted, their resultant 3D physical property models are stitched together. This workflow is often inadequate for very large gravity and/or magnetic surveys.
As data acquisition systems continue to evolve for geophysical surveys that will continue to increase in the volume of geophysical data being acquired, and interpreters demand accurate and higher resolution 3D volume imaging of the subsurface from said geophysical data, there exists an urgent need for robust methods to produce 3D volume images from geophysical data.
Subsurface imaging using geophysical methods has applications beyond resource exploration and production. The development and use of high-resolution airborne and satellite surveillance has prompted the widespread proliferation of covert tunnels and underground facilities (UGFs). UGFs are used to produce and harbor both weapons and illegal drugs, and in the case of tunnels, move contraband and people without detection across international borders. There exists an urgent need for 3D volume imaging of geophysical data for tunnel and UGF detection and monitoring.
Geophysical methods have been developed for detecting unexploded ordinance (UXO) that contains metal and/or electronic parts. While UXO detection is relatively mature discipline for weapons test site remediation, improvised explosive devices (IEDs) made primarily from fertilizer and lacking metal or electronic parts represent a persistent direct threat to civilian and military personnel in combat zones. IEDs are far more difficult to detect than standard UXO. There exists an urgent need to develop 3D volume imaging of geophysical, data measured from UAVs and vehicles for IED detection and discrimination.